Starting device with a ptc-resistor for a single phase motor

ABSTRACT

This invention relates to a single phase motor which includes in the circuit means thereof a starting winding in series with a PTC-resistor having lower and upper resistance ranges. The PTC resistor is designed so that it is only operable in the low resistance range when heated solely by the effects of the starting current flowing therethrough. External heating means is provided for supplying an amount of auxiliary heat to the PTCresistor to effect a shift to the high resistance range. The external heating means supplies heat at a rate which causes the shift to the higher resistance range to occur at a predetermined time.

United States Patent [72] Inventor Svenl-"orst CostaMesa.Calll. 21 AppLNo. 760,780 221 Filed Sept.19,l968

[45] Patented July 13, 1971 [73] Assignee Danioss AIS Nordborg, Denmark [$4] STARTING DEVICE WITH A FTC-RESISTOR FOR A SINGLE PHASE MOTOR 3 Chims,2 Drawing Figs.

[52] US. 318/221 E,v

318/221 H, 318/229 [51] Int. "l 1102p 1/44 [50] Field of Search 318/220. 22 l 229 [56] References Cited FOREIGN PATENTS 1,042,126 9/1966 GreatBritaini Primary Examiner-Oris L. Rader Assistant Examiner-Gene Z. Rubinson Anorney-wayne B. Easton rate which;causes the shift to the higher resistance range to occur at a predetermined time.

THERMAL INSULATION HEATlNG RESISTOR PATENTEflJuualsn 3,593,081

THERMAL INSULATION 2 PTC RESISTOR hitherto located in a range of lower resistance, moves into a range of higher resistance beyond the peak of its current-voltage curve. Such design of the FTC-resistor causes difficulties, however, if fairly lengthy starting times, seconds, for example, have to be considered. In this case an inert PTC-resistor is required which, because of its greater dimensions, is thermally loaded in a nonuniform manner and easily cracks.

The object of the invention is to provide a starting device wherein the starting time can be selected independently of the size of the FTC-resistor, and which, in particular, is suitable for fairly lengthy starting periods.

According to the invention, this object is achieved by so dimensioning the FTC-resistor that, upon its being heated up by the starting current only, its operating point is still located .in front of the peak of its current-voltage curve, and by providing an additional heating device which shifts the operating point beyond the peak at the desired moment. In this arrangement a smaller FTC-resistor can be used which is heated up rapidly and uniformly by the current flow- 'ing therethrough and therefore does not run the risk of damage due to mechanical thermal stresses. It reaches its equilibrium temperature after a short timeand independently of the required starting period. The change-over" to the range of higher resistance only takes place with the help of the additional heating device. At a freely selectable point of time, this latter device supplies additional energy such that the increase in temperature necessary for surmounting the peak is supplied. Since the FTC-resistor, due to self-heating, can already have reached an operating point shortly in front of the peak, a very small difference in temperature suffices for this.

The precise point in time at which the additional heating device becomes operative can be fixed in a large number of ways. For example, the heating device can be supplied with a time-controlled heating impulse. The heating device can also be switched on when the motor is switched on, but can act upon the FTC-resistor by way of an action-delaying path. For instance, thermal insulation between the heating device and the FTC-'resistor can serve to provide the delayed action. lt is also possible, without difficulty, to design a heating resistor of such mass and inertia that the desired delayed action takes place.

In a further form of the invention, the additional heating device can remain continuously switched on while the motor I is operating. By this step it is ensured that in all circumstances the PTC-resistor retains a temperature in the high ohmic 'range even after the current passing therethrough has been reduced.

The invention will now be described in more detail by reference to an embodiment illustrated in the drawing, wherein:

FIG. I shows the current-voltage characteristic curve of a FTC-resistor having different operating points, and

intersection, of which, however, only points P1 in the lowohmic range and P in the high-ohmic range are stable,

whereas point P is not stable.

When the FTC-resistor is heated by the current flowing therethrough, the branch of the curve will run from the zero point to the operating point P,. The FTC-resistor thus remains in the low-ohmic range. This is in contrast to the known arrangements wherein the FTC-resistor was so designed that no stable operating point P, at all established itself; rather, the peak S of the curve I lay below the operating characteristic curve II.

By means of addiiional heating, the temperature of the FTC-resistor is so raised at the desired time, i.e. at the end of the starting period, that the operating point shifts beyond the peak S. As soon as this is achieved, the operating point automatically moves to P; in the high-ohmic range.

In the connecting diagram shown in FIG. 2, a single-phase motor 1 with a winding 2 for normal running and a starting winding 3 is connected via a switch 4 to the terminals 5 and 6 of an A.C. mains supply. A FTC-resistor 7 is connected in series with the starting winding. Connected in parallel with this arrangement is an additional heating resistor 8 which acts on the FTC-resistor 7 by way of a thermal insulation 9.

When the switch 4 is thrown, a large current first flows in the starting branch. This leads to rapid heating up of the PTC- resistor 7. A condition of equilibrium is established at a point P however, wherein the starting winding 3 is supplied with a sufficiently large starting current. From the outset, the heatin g resistor 8 is also heated by a constant current. It warms up gradually and, delayed by its mass and by the thermal insulation 9, gives off heat to the FTC-resistor 7. By correctly rating the elements 8 and 9, it is possible, without difficulty, to cause the additional heat to act upon the FTC-resistor only towards the end of the selected period, e.g. after 10 seconds, and in such a manner that the peak Scan be surmounted. At this moment, the PTC-resistor switches over to point P so that the starting current is reduced to a low value that causes no damage during continuous operation. The additional heating resistor 8 remains switched on during the entire running period of the motor and ensures that the temperature remains at a value necessary for keeping the current in the starting winding small. In particular, the starting current can be kept lower than would be possible without the resistor 8 being continuously switched in.

In many cases it also suffices if the heating resistor 8 is switched in for only a brief time, i.e. towards the end of the starting period. The heating resistor can also have a variable size, for example the current from the winding for normal running can flow through it, or a current which in turn flows through the FTC-resistor, this being achieved, for example, by the heating resistor being in parallel with the starting winding.

I claim:

I. A single phase motor including circuit means, a branch of said circuit means having winding means for starting and a FTC-resistor, said branch being connected to receive a starting current, said FTC-resistor having temperature dependent low and high resistance ranges and being designed to be operable in said low resistance range when heated solely by the effects of said starting current flowing therethrough, external heating means for supplying auxiliary heat to said PTC-resistor, and heat transfer curtailing means for initially curtailing the transfer of heat from said heating means to said resistor to effect a sudden shift to said high resistance range at a predetermined time.

2. A motor according to claim 1 wherein said external heating means supplies heat to said FTC-resistor continuously during the operation of said motor. 3. A motor according to claim 1 wherein said heat curtailing means is thermal insulation material between said heating means and said PTC resistor. 

1. A single phase motor includiNg circuit means, a branch of said circuit means having winding means for starting and a PTCresistor, said branch being connected to receive a starting current, said PTC-resistor having temperature dependent low and high resistance ranges and being designed to be operable in said low resistance range when heated solely by the effects of said starting current flowing therethrough, external heating means for supplying auxiliary heat to said PTC-resistor, and heat transfer curtailing means for initially curtailing the transfer of heat from said heating means to said resistor to effect a sudden shift to said high resistance range at a predetermined time.
 2. A motor according to claim 1 wherein said external heating means supplies heat to said PTC-resistor continuously during the operation of said motor.
 3. A motor according to claim 1 wherein said heat curtailing means is thermal insulation material between said heating means and said PTC resistor. 